The invention relates to testing isolated buried thermoplastic pipes for leakage.
Leakage detection of distribution pipe is performed by measuring any pressure decay rate during a pneumatic pressure test. The installation is pressurised to 1.5 times its maximum working pressure (typically 3, 6 or 7 bargauge), and regular pressure readings are taken with time until the engineer is satisfied that there is no significant leak.
The pass/fail criterion applied within British Gas plc is that the significant leak is one that represents a leakage of 0.0028 standard cubic meters per hour of gas at the maximum working pressure of the main.
For constant temperature and internal volume, the pressure drop rate caused by a leak is proportional to the leak flowrate, and inversely proportional to the internal volume of the pipework under test. In large volumes very small drop rates have to be resolved. For example, in 100 cubic meters at a 6 bar test, the pass fail drop rate is 36 microbars per hour. Using conventional instrumentation, for example capable of the order of millibar resolution at 7 bar gauge, several days duration of test are required in order to register such small drop rates.
During such extended test times, other influences such as changing ambient temperature have to be accounted for both from the instrumentation drift and test pressure change viewpoints.
For modern polyethylene gas pipe systems, in addition to temperature change, the other major influence on pressure drop rate behaviour is the creep behaviour of the pipe when pressurised, which causes a time dependent volume increase, in turn leading to a further pressure drop.
For a system with no leak and at constant temperature, the pressure drop behaviour caused by creep is independent of pipe volume, but depends upon the material grade, the pressure (stress), the Standard Dimension Ratio (SDR) (which is the outside diameter of the pipe divided by the wall thickness) and the recent stress history of the pipe. The drop rate curve is exponential in nature, eventually settling to a constant pressure as the volume changes become diminishingly small. Hence for small test volumes the drop rate due to the significant leak dominates over the drop rate due to creep, but in larger volumes the drop rate due to creep dominates in the early days of the test.